The liquid crystal displays (LC displays) used at present are usually those of the TN (“twisted nematic”) type. However, these have the disadvantage of a strong viewing-angle dependence of the contrast.
In addition, so-called VA (“vertically aligned”) displays are known which have a broader viewing angle. The LC cell of a VA display contains a layer of an LC medium between two transparent electrodes, where the LC medium usually has a negative value of the dielectric (DC) anisotropy. In the switched-off state, the molecules of the LC layer are aligned perpendicular to the electrode surfaces (homeotropically) or have a tilted homeotropic alignment. On application of an electrical voltage to the two electrodes, a realignment of the LC molecules parallel to the electrode surfaces takes place.
Furthermore, so-called FFS (“fringe-field switching”) displays have been reported (see, inter alia, S. H. Jung et al., Jpn. J. Appl. Phys., Volume 43, No. 3, 2004, 1028), which contain two electrodes on the same substrate, one of which structured in a comb-shaped manner and the other is unstructured. A strong, so-called “fringe field” is thereby generated, i.e. a strong electric field close to the edge of the electrodes, and, throughout the cell, an electric field which has both a strong vertical component and also a strong horizontal component. FFS displays have a low viewing-angle dependence of the contrast. FFS displays usually contain an LC medium with positive dielectric anisotropy, and an alignment layer, usually of polyimide, which provides planar alignment to the molecules of the LC medium.
FFS displays can be operated as active-matrix or passive-matrix displays. In the case of active-matrix displays, individual pixels are usually addressed by integrated, non-linear active elements, such as, for example, transistors (for example thin-film transistors (“TFTs”)), while in the case of passive-matrix displays, individual pixels are usually addressed by the multiplex method, as known from the prior art.
Furthermore, FFS displays have been disclosed (see S. H. Lee et al., Appl. Phys. Lett. 73(20), 1998, 2882-2883 and S. H. Lee et al., Liquid Crystals 39(9), 2012, 1141-1148), which have similar electrode design and layer thickness as FFS displays, but comprise a layer of an LC medium with negative dielectric anisotropy instead of an LC medium with positive dielectric anisotropy. The LC medium with negative dielectric anisotropy shows a more favorable director orientation that has less tilt and more twist orientation compared to the LC medium with positive dielectric anisotropy, as a result of which these displays have a higher transmission.
However, the use of LC media with negative dielectric anisotropy in FFS displays has also several drawbacks. For example, they have a significantly lower reliability compared to LC media with positive dielectric anisotropy.
The term “reliability” as used hereinafter means the quality of the performance of the display during time and with different stress loads, such as light load, temperature, humidity, voltage, and comprises display effects such as image sticking (area and line image sticking), mura, yogore. etc. which are known to the skilled person in the field of LC displays. As a standard parameter for categorizing the reliability usually the voltage holding ration (VHR) value is used, which is a measure for maintaining a constant electrical voltage in a test display. The higher the VHR value, the better the reliability of the medium
The reduced reliability of an LC medium with negative dielectric anisotropy in an FFS display can be explained by an interaction of the LC molecules with the polyimide of the alignment layer, as a result of which ions are extracted from the polyimide alignment layer, and wherein LC molecules with negative dielectric anisotropy do more effectively extract such ions.
This results in new requirements for LC media to be used in FFS displays. In particular, the LC medium has to show a high reliability and a high VHR value after UV exposure. Further requirements are a high specific resistance, a large working-temperature range, short response times even at low temperatures, a low threshold voltage, a multiplicity of grey shades, high contrast and a broad viewing angle, and reduced image sticking.
Thus, in displays known from prior art often the undesired effect of so-called “image sticking” or “image burn” is observed, wherein the image produced in the LC display by temporary addressing of individual pixels still remains visible even after the electric field in these pixels has been switched off, or after other pixels have been addressed.
This “image sticking” can occur on the one hand if LC media having a low VHR are used. The UV component of daylight or the backlighting can cause undesired decomposition reactions of the LC molecules therein and thus initiate the production of ionic or free-radical impurities. These may accumulate, in particular, at the electrodes or the alignment layers, where they may reduce the effective applied voltage.
Another problem observed in prior art is that LC media for use in displays, including but not limited to FFS displays, do often exhibit high viscosities and, as a consequence, high switching times. In order to reduce the viscosity and switching time of the LC medium, it has been suggested in prior art to add LC compounds with an alkenyl group. However, it was observed that LC media containing alkenyl compounds often show a decrease of the reliability and stability, and a decrease of the VHR especially after exposure to UV radiation.
It is therefore an object of the present invention to provide improved LC media for use in FFS displays, in particular those of the UB-FFS mode, which do not exhibit the disadvantages described above or only do so to a small extent and have improved properties. A further object of the invention is to provide FFS displays with good transmission, high reliability, a VHR value especially after UV exposure, a high specific resistance, a large working-temperature range, short response times even at low temperatures, a low threshold voltage, a multiplicity of grey shades, high contrast and a broad viewing angle, and reduced image sticking.
Upon further study of the specification and appended claims, other objects, aspects and advantages of the invention will become apparent.
These objects are achieved in accordance with the present invention by providing an FFS display and LC media used therein as described and claimed hereinafter. In particular, the inventors of the present invention have found that the above objects can be achieved by using an LC medium comprising a di- or multireactive polymerizable compound as described hereinafter, and preferably comprising one or more alkenyl compounds, in a FFS display. It has also been found that when using such di- or multireactive compounds in an LC medium for use in an FFS display, surprisingly the reliability and the VHR value after UV exposure are higher, compared to an LC medium without a polymerizable compound.
Also, the use of an LC medium comprising a di- or multireactive polymerizable compound as described hereinafter allows to exploit the known advantages of alkenyl-containing LC media, like reduced viscosity and faster switching time, and at the same time leads to improved reliability and high VHR value especially after UV exposure.